Precipitation-hardenable stainless steel

ABSTRACT

Precipitation-hardenable stainless steel combining strength and corrosion-resistance and method of hardening the same. The steel contains the four essential ingredients chromium, cobalt, molybdenum and copper, with remainder iron, the chromium amounting to about 11 to 18 percent, the cobalt about 5 to 14 percent, the molybdenum about 1 to 9 percent, and the copper about 1 to 5 percent. Neither the carbon nor the nitrogen content should exceed about 0.15 percent. Where desired, tungsten and/or vanadium may be partially substituted for molybdenum, this in amounts up to about 6 percent. As annealed, the steel is martensitic. Hardening from the annealed condition is had by heating at descending or cascading precipitation temperatures, i.e., 1,000* to 1,300* F. and then 850* to 1,000* F. In precipitation-hardened condition it is suited to a wide variety of applications in the food-handling, petro-chemical and aircraft industries.

United States Patent 1191 Tanczyn 1 1 PRECIPITATION-HARDENABLE STAINLESS STEEL [75] Inventor: Harry Tanczyn, Baltimore, Md.

[73] Assignee: Armco Steel Corporation,

Middletown, Ohio [22] Filed: Aug. 29, 1969 [21] Appl. No.: 854,253

[52] US. Cl. ..75/l25, 75/126 H, 75/126 C,

75/128 B [51] Int. Cl ..C22c 39/54 [58] Field of Search ..75/l25, 126 H [56] References Cited UNITED STATES PATENTS 2,905,577 /1959 Harris ..75/126 H 2,908,565 10/1959 Nelson ..75/125 1,998,953 4/1935 Emmons... .....75/l26 H 2,462,665 2/1949 Olcott ..75/l26 H 2,590,835 4/1952 Kirkby ..75/126 H 2,932,568 4/1960 Kegerise ..75/l25 1 March 6, 1973 Primary ExaminerHyland Bizot Att0rneyJohn Howard Joynt 5 7] ABSTRACT Precipitation-hardenable stainless steel combining strength and corrosion-resistance and method of hardening the same. The steel contains the four essential ingredients chromium, cobalt, molybdenum and copper, with remainder iron, the chromium amounting to about 11 to 18 percent, the cobalt about 5 to 14 percent, the molybdenum about 1 to 9 percent, and the copper about 1 to 5 percent. Neither the carbon nor the nitrogen content should exceed about 0.15 percent. Where desired, tungsten and/or vanadium may be partially substituted for molybdenum, this in amounts up to about 6 percent. As annealed, the steel is martensitic. Hardening from the annealed condition is had by heating at descending or cascading precipitation temperatures, i.e., 1,000 to 1,300 F. and then 850 to 1,000 F. In precipitation-hardened condition it is suited to a wide variety of applications in the food-handling, petro-chemical and aircraft industries.

5 Claims, No Drawings PRECIPITATION-HARDENABLE STAINLESS STEEL My invention is concerned with the precipitationhardenable stainless steels.

Among the objects of the invention is the provision of a stainless steel which readily lends itself to conversion in the hot-mill, as in the production of billets, blooms, plate, rod, and the like, and to subsequent conversion, where desired, by cold-working into plate, sheet, strip and the like, as well as cold-drawn rod and wire; which steel in one condition of heat-treatment, that is, in the annealed or solution-treated condition, readily may be worked, formed or otherwise fabricated into a wide variety of articles and products as by bending, cutting, shearing, blanking, punching, threading or otherwise machining, by a customer-fabricator; and which steel, as fabricated, then may be readily hardened and strengthened by precipitation heat-treating methods for service in the form of tanks, vats, valves and conduits in the petro-chemical and foodhandling industries where elevated temperatures are encountered, these approaching l,l or 1,200 F., and actuators, fastening devices, landing gear parts and the like in aircraft industries.

Other objects of my invention in part will become apparent during the course of the description which follows and in part will be especially pointed to.

The invention, then, may be considered to reside in the combination of elements, the composition of ingredients, the several operational steps, and in the relation between the same, all as more particularly described herein and particularly set out in the claims at the end of this specification.

CROSS-REFERENCE TO RELATED APPLICATIONS In general, the present application for patent may be considered as related to my copending companion applications, Ser. Nos. 854,252 and 854,254, respectively titled Precipitation-Hardenable Martensitic Stainless Steel and Method, and Precipitation-Hardenable Austenitic Stainless Steel and Method, both filed of even date herewith.

BACKGROUND OF THE INVENTION To gain a better understanding of certain features of my invention, it may be well to note at this point that the precipitation-hardenable stainless steels are rather well known in the art. While some are austenitic, or semi-austenitic, in the annealed condition and require a double heat-treatment to effect a hardening, others are martensitic in the annealed condition and are hardened by a mere heating at precipitation temperatures. An illustration of the latter is the Armco 17-4 PI-I steel (about 16.5 percent chromium, about 4 percent nickel, about 4 percent copper, and remainder iron). This steel in the precipitation-hardened condition combines excellent corrosion-resistance with high strength, the ultimate tensile strength being on the order of some 200,000 psi. The steel is suited to applications up to about 600 F. lt forms the subject of the Clarke U.S. Pat. No. 2,482,096 of Sept. 20, 1949. Another chromium-nickel steel but containing aluminum (about 16 percent chromium, about 7 percent nickel, about 0.8 percent aluminum, and remainder iron) also is martensitic in the annealed or solution-treated condition and hardens by mere heating at precipitation temperatures. This steel, too, enjoys excellent corrosion-resistance, along with good strength in the precipitation-hardened condition. Here the ultimate tensile strength amounts to about 185,000 psi. It is suited to applications involving temperatures up to about 900 F. The steel forms the subject of the Goller U.S. Pat. No. 2,505,762 of May 2, 1950.

Although the two steels noted enjoy an excellent combination of properties, there nevertheless are many applications where the corrosion-resistance is inadequate and, indeed, where even greater strength is required and at somewhat higher temperatures. Unfortunately, both steels are inclined to scale somewhat during conventional hot-working operations; scale removal requires an extra operational step, this at extra cost.

A stainless steel containing the ingredients chromium, nickel, cobalt, molybdenum and copper, the same four alloying ingredients employed in my steel, forms the subject of the Clarke U.S. Pat. No. 2,536,034 of Jan. 2, 1951. That steel additionally contemplates the presence of carbon, titanium and columbium. And that steel is substantially fully austenitic under all conditions of heat-treatment; there is no transformation to martensite. As a result, the room-temperature strength which is had only amounts to some 100,000 psi, a figure wholly inadequate for many applications.

While the several chromium-nickel stainless steels discussed above are characterized by a number of highly desirable physical properties, none enjoys superior strength, especially at elevated temperatures exceeding 900 or 1,000 F., along with toughness and ductility.

SUMMARY OF THE INVENTION In the steel of my invention I overcome many of the deficiencies of the steels of the prior art and combine good workability in the solution-treated condition of the metal with great strength, toughness and corrosionresistance in another condition, that is, in the precipitation-hardened condition. My steel essentially contains the four ingredients chromium, cobalt, molybdenum and copper, with remainder principally iron. The chromium content amounts to about 11 percent to about 17 or 18 percent, the cobalt about 5 or 6 percent to about 11, '12 or even 14 percent, the molybdenum about 1 or 3 percent to about 8 or 9 percent, and the copper about 1 or 2 percent to about 4 or 5 percent. The sum of the chromium and cobalt contents should not exceed about 25 percent. Where desired, tungsten and/or vanadium may be partially substituted for molybdenum, this on a 1:1 basis amounts up to about 6 percent in total, particularly about 1 percent to about 6 percent.

In my steel carbon and nitrogen, of course, are commonly present, each in amounts not exceeding 0.15 percent, and preferably not exceeding 0.10 percent. With the exercise of care in the selection of materials and in melting, both the carbon and nitrogen contents may be maintained at figures not exceeding about 0.05

percent. Where desired, columbium may be present,-

While there may be some nickel present as an impurity, this ingredient should be maintained well below 1 percent, and preferably is maintained well below 0.5 percent, this generally amounting to less than 0.25 percent. Nickel, I find, is the ingredient which is principally responsible for limiting the temperatures at which steel is suited for use; nickel is the ingredient which lowers the temperature at which martensite will revert to austenite and hence the upper operating temperature. Nickel is objectionable in my steel because of the limiting effect on operating temperatures.

Now the composition of the steel is in every sense critical, for I find that where the prescribed combination of the ingredients chromium, cobalt, molybdenum and copper is maintained, the desired combination of properties is had. Where, however, one or more of the assigned limits of these ingredients is substantially departed from, one more of the desired properties is lost or severely sacrificed; in any event, the desired combination of properties no longer is had.

The chromium content of my steel must be at least about 1 1 percent, and preferably at least about 13 per cent, for otherwise the corrosion-resisting characteristics of the metal become inadequate. But the chromium content should not exceed about 18 percent, and preferably should not exceed about 17 percent, because of the strong inclination toward ferrite formation, that is, the formation of delta-ferrite.

The cobalt content must be at least about 5 percent, and preferably should be at least about 6 or 7 percent, in order to insure a freedom from delta-ferrite. I find that cobalt is most effective in overcoming the ferriteforming tendency of the chromium content, this without lowering the temperature at which the martensite formed during the aging treatment is inclined to revert to austenite. Actually, it is my view that cobalt substantially raises the temperature at which martensite will revert to austenite and substantially increases the temperatures of useful application of the steel as noted above. A cobalt content exceeding 14 percent, however, is not acceptable because I find that with an excessive cobalt content, when taken with a substantial chromium content, that is, where the sum of the chromium and cobalt contents exceeds about percent, there is formed a brittle chromium-cobalt phase which seriously and adversely affects the mechanical properties of the metal. The critical limit on the sum of the chromium and cobalt contents is particularly felt in my steel, where there is not had the ameliorating effects of nickel on the embrittling tendencies.

In my steel the molybdenum content must be at least about 1 percent, and preferably some 3 or 4 percent, in order that there may be had the desired strength and resistance to corrosion, for it is my view that the desired mechanical properties in substantial measure may be attributed to the formation of a cobalt-molybdenum compound, or perhaps a compound of chromium, cobalt and molybdenum, or even a chromiumcobalt-molybdenum-copper ingredient. A molybdenum content exceeding about 8 or 9 percent, however, is undesirable because of the ferrite-forming tendencies of molybdenum, and because of the hot-working difficulties encountered with an excess of this ingredient.

Copper, while essential to my steel, this in the amount of at least about 1 percent and for best results about 2 percent, should not exceed some 4 or 5 percent. Copper lends stability to the metal, at least about 1 percent being necessary for this purpose. But copper exceeding 5 percent is undesirable because this exceeds the copper-solubility level. While it is thought, as noted above, that copper very well may combine with cobalt and molybdenum, or indeed, chromium, cobalt and molybdenum, to form the principal hardening and strengthening factor, this is a theoretical explanation by which I do not care to be bound. Suffice it to say that copper in the amounts indicated, this along with the other ingredients in the amounts indicated, is essential to the properties sought and had in the steel of my invention.

The steel may be melted in the electric arc furnace or, where desired, melted in the vacuum furnace, or, indeed, by way of a combination of the two.

The steel, however melted, is cast into ingots, which by conventional hot-mill methods are converted into blooms, billets, bars, rod, wire, plate, sheet and strip. The metal also may be made available in the form of forging billets, and in the form of castings. Further conversion of bloom, billet, bar, rod, wire, plate and sheet may be had through conventional cold-working operations, these yielding cold-rolled plate, sheet and strip, as well as cold-drawn rod and wire.

The mill products generally are supplied a customerfabricator in the cold-rolled or cold-drawn condition. Where desired, however, they may be supplied in the solution-treated or annealed condition, that is, in the condition following a heating and cooling in air, oil or water from some 1,500 to 2,100F. The metal well lends itself to forming and fabrication, as by pressing, bending, cutting, machining, threading and the like, as in the production of a wide variety of articles of ultimate use. And following fabrication, or prior to, as desired, the metal is hardened and strengthened by precipitation-hardening heat-treatment.

Now the steel of my invention in the annealed or solution-treated condition, that is, following the heating at solution-treating temperatures and cooling, is martensitic as noted above. The precipitation-hardening effect is achieved by merely heating the metal at some 850 to 1,300 F.

I find'that perhaps greatest hardness and strength are had by subjecting my steel to a cascade system of heathardening. In this, following a heating at solution-treating temperatures and cooling, the metal is subjected to a first heating at some l,000 to l,300 F. to effect a first aging. Then there is had a further heating, this at a lower temperature, namely, some 850 to 1,000 F., to effect a second aging. In my view, the higher temperatures of the first aging treatment afford some relief from the stresses encountered in heat-treatment. This relief of stresses in the martensite is such as to permit the further hardening by further heating at the somewhat lower aging temperatures. Best results are had where the steel is first aged at the more limited temperature range of 1,050" to l,l50 F. and further aged at the more limited range of 925 to 975 F. The strength had approaches 300,000 psi.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While, as noted above, the precipitation-hardenable steel of my invention, in broad aspect, essentially consists of about 11 percent to about 18 percent chromium, about 5 percent to about 14 percent cobalt, about 1 percent to about 9 percent molybdenum, about 1 percent to about 5 percent copper, and remainder substantially all iron, there, however, are a number of specific steels in which there is enjoyed a best combination of properties.

One of the preferred steels of my invention essentially consists of about 11 percent to about percent chromium, about 8 percent to about 12 percent cobalt, about 4 percent to about 8 percent molybdenum, about 2 percent to about 4 percent copper, with carbon not exceeding about 0.10 percent, nitrogen not exceeding about 0.10 percent, with columbium up to 0.50 percent, and remainder substantially all iron. The nickel content of the steel is less than 0.50 percent. This steel enjoys an excellent combination of hot-workability in the mill, good cold-workability and good fabricating properties, along with good hardness and strength, these latter when in the age-hardened or precipitationhardened condition. The corrosion-resistance in hardened condition is good. 7

A steel enjoying somewhat better corrosion-resistance essentially consists of about 13 percent to about 15 percent chromium, about 8 percent to about 12 percent cobalt with the sum of the chromium and cobalt contents not exceeding about 25 percent, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, with carbon not exceeding 0.05 percent and nitrogen not exceeding 0.10 percent, and remainder substantially all iron. Here again, oclumbium may be employed in amounts up to about 0.50 percent or more. I A further steel essentially consists of about 11 percent to about 13 percent chromium, about 9 percent to about 11 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, about 0.01 percent to about 0.06 percent carbon, nitrogen up to about 0.06 percent, up to about 0.50 percent columbium, and remainder substantially all iron. This steel is free of delta-ferrite and enjoys best isotropic properties. It is particularly suited to the production of fasteners, that is, bolts, studs and the like, because of excellent transverse properties, most fasteners being loaded in shear; the shear strength amounts to at least 165,000 psi as against some 150,000 psi for the titanium alloys.

A steel enjoying maximum corrosion-resistance in the precipitation-hardened condition, along with good workability in the annealed condition, essentially consists of about 15 percent to about 17 percent chromium, about 6 percent to about 10 percent cobalt with the sum of the chromium and cobalt contents not exceeding about 25 percent, about 3 percent to about 5 percent molybdenum, about 2 percent to about 4 percent copper, carbon up to about 0.10 percent, nitrogen up to about 0.06 percent, up to about 0.50 percent columbium, and remainder substantially all iron.

In further illustration of the steels of my invention, 1 give below in Table I the composition of some 10 specific steels. In Table 11(a) 1 set out the mechanical I properties of these steels in the solution-treated condition. And in Tables 11(b) and 11(c) I give the mechanical properties of certain of these steels in heat-hardened condition, that is, when hardened by single precipitation treatment and when hardened by double or cascade precipitation treatment.

Table I Chemical Composition of Ten Steels According to the Invention Heat No. C Cr Ni Mo Co Cu Cb N R 7004 .015 12.76 .21 5.16 10.30 3.05 .17 .020 R7006 .015 14.03 .20 5.18 10.00 3.03 .18 .019 R 7009 .054 13.80 .26 5.12 9.80 3.05 .19 .067 R7014 .014 12.78 .29 5.10 9.99 3.01 .19 .057 R7015 .056 12.78 .28 4.95 10.10 3.03 .21 .057 R 7016 .019 13.97 .27 5.11 9.92 3.06 .19 .054 VR112 .050 12.50 .29 5.00 10.00 3.00 .33 .015 VR 113 .048 12.60 .27 4.80 9.40 2.90 .36 0.14 VR 114 .040 11.60 .27 4.77 .43 2.92 .35 .014 VR 115 .024 12.5 .27 4.77 9.39 2.93 .34 .017

For the R heats, manganese about 1%, silicon about .4%, phosphorus about 020% and sulfur about .015%; for the VR heats, manganese about 0.4%, silicon about .10%, phosphorous about 005% and sulfur about 005%.

Table II(a) Mechanical Properties of the Steels of Table 1 in Solution- Treated Condition Ultimate 0.2% Yield Strength Strength Red. Heat No. (psi) (psi) Elong. of Area R 7004 141,000 101,200 16.2 54.0 R 7006 143,600 103,000 15.5 53.0 R 7009 144,500 102,600 16.0- 52.0 R 7014 142,700 99,400 17.2 56.0 R 7015 147,200 105,200 15.8 52.0 R 7016 146,500 104,800 15.4 53.0 VR 112 147,000 103,000 16.0 54.0 VR 113 145,100 101,000 16.5 55.0 VR 114 143,400 99,800 16.2 55.0 VR 115 140,200 99,600 16.8 56.0

2000F. for 20 minutes and oil quench 2000F. for 20 minutes and oil quench refrigeration at "F. for 4 hours The mechanical properties of the steels of Table I when hardened subsequent to the solution-treatment are set out below in Table (12). Here the steels were solution-treated at a temperature of 2,000 F. for 20 minutes and oil quenched, or quenched and refrigerated as the case may be, to effect transformation, following which they were reheated at 1,050 F. for 2 hours and air cooled. The properties reported on are ultimate tensile strength, 0.2 percent yield strength, percent elongation, and percent reduction of area.

A study of the mechanical test data presented in Tables 11(a) and 1l(b) above, this with respect to the steels the chemical compositions of which are set out in Table 1, immediately reveals the great increase in ultimate tensile strength and yield strength had by virtue of the precipitation-hardening treatment; At the same time there is revealed in Table ll(a) the excellent ductility had in the solution-treated condition. It will be seen that with solution-treatment at 2,000 F. and mere oil quenching, the four steels so treated have an elongation of some 16 to 1 percent and a reduction of area of some 54 to 56 percent. With the further refrigeration treatmerit resorted to for the six further steels, the elongation and reduction of area figures are about the same.

And all ten steels in solution-treated condition, with or 1 without refrigeration, are characterized by tensile strengths of some 140,000 to 147,000 psi andyield strengths of some 99,000psi to 105,000 psi, with no special benefit being noted with the refrigeration treat ment.

Consideration of the mechanical test data presented in Table 11(b) for the steels in precipitation-hardened condition as related to that presented in Table ll(a) for the steels in solution-treated condition immediately reveals the great increase in strength deriving from the precipitation-hardening treatment. Actually, it will be seen that both the tensile. strength and the .yield strength are virtually doubled with that treatment.

Moreover, study of the mechanical test dataset out in Tab1e'll(b), when related to the chemical composition of the steels tested, this as set out in Table I, rather clearly reveals that best strength is enjoyed by the steels having a chromium content somewhat on the low side, that is, about 13 percent, with one or both of carbon and nitrogen on the high side, that is, about 0.05 percent carbon and about 0.06 percent nitrogen. Thus, the Heat No. R 7014, having a chromium content of 12.78 percent, carbon content of 0.014 percent and a nitrogen content of 0.057 percent, is seen to have a ten sile strength of about 286,000 psi, this as against the Heat No. R 7006, having a chromium content of 14.03 percent, a carbon content of 0.015 percent and a nitrogen content of 0.019 percent, with a tensile strength of 229,000 psi. The superior strength had in the steels of the somewhat limited chromium content and the somewhat greater carbon and nitrogen contents I attribute to a freedom from delta-ferrite.

Although all of the steels of Table I enjoy a tensile nitrogen balance (Heat Nos. R 7009, R 7014 and VR 1 l4) enjoy a strength exceeding 280,000 psi, this along with good ductility. Best results in matters of a combination of strength and ductility are had in the steels of the lower manganese, silicon, phosphorus and sulfur contents (Heat Nos. VR 112, VR 113, VR 114 and VR 115). As'previously noted, however, it is the steel with chromium somewhat on the low side and carbon somewhat on the high (Heat No. VR 1 14) which enjoys best strength, along with good ductility.

The mechanical properties of certain of the steels of Table I are reported below in Table ll(c), these for six steels treated at 2,000" F. for 20 minutes and oil quenched and refrigerated, followed by a first aging treatment at 1,050" F. for 2 hours and air cooling, followed by a further aging at 950 F. for 1 hour and air cooling.

Greatest strength is achieved in my steels through the double-aging or cascade-aging'treatment,that is, first strength well in excess of 220,000 psi, the best steels,

those of the lower chromium and higher carbon and/or aging the solution-treated steel at some 1,050 F, and then subsequently again aging the steel, this at the somewhat lower temperature of 950 F. This readily may be seen by comparing the test data reported in Table 11(0) for the double-aged steel with that reported in Table 11(b) for the single-aged steel. Thus, the steel of the somewhat higher chromium content and lower carbon and nitrogen contents (Heat'No. R 7006), having an ultimate tensile strength of 229,000 psi in single precipitation-hardened condition, is seen to possess a strength at 257,000 psi in double precipitationhardened condition, a gain of almost 30,000 psi. The is matched by a gain in yield strength of 25,000 psi (194,000 psi with single-aged treatment and2l9,000 psi with double-aged treatment). The steel of the somewhat lower chromium content and higher carbon and/or nitrogen content (Heat No. R 7014), which in the single-aged condition is seen to have an ultimate tensile strength of 286,000 psi, is found to acquire a strength of 294,000psi with the further aging treatment, a gain here of 8,000 psi.

It appears, then, that with the double-aging treatment, not only is there an improvement in strength, but also there is a trend toward equalizing minor compositional difierences to gainv maximum strength,- this without sacrifice of ductility. I attribute the equalizing effect of the double-aging treatment to an elimination of delta-ferrite with the further aging, although lprefer not to be bound by this explanation. Suffice it to say that with the'double or cascadeaging treatment, there is had maximum strength, along with good retained ductility.

While it is the steels of the higher chromium contents which enjoy greatest corrosion-resistance, it is the steels of the lower chromium contents which, in the solution-treated condition, enjoy best formability and in the hardened condition, greatest strength. But all of the steels work well in the solution-treated condition, and all are strong in the hardened condition. It is the lower chromium steels, and of somewhat higher carbon and/or nitrogen contents, which are perhaps best suited to applications in the aircraft industries because of the greater strength and the high strength-to-weight ratio which they enjoy. The steels of the higher chromium contents are perhaps best adapted to the petro-chemical and food-handling applications because of better corrosion-resistance.

In conclusion, then, it will be seen that I provide in my invention a precipitation-hardenable stainless steel, and method of hardening the same, in which there is realized the various objects set out above, this together with many practical advantages. In my steel there can be developed great strength, this approaching some 300,000 psi. The steel in hardened condition is particularly characterized by a combination of strength and corrosion-resistance. Moreover, it is suited to applications at temperatures significantly higher than the known and commonly used precipitation-hardened stainless steels; the preferred steels can withstand operating temperatures approaching l,100 or 1,200 F.

Inasmuch as many embodiments may be made of my invention, and many changes made in the several embodiments set out above, it is to be understood that all matter described herein is to be interpreted as illustrative and not by way of limitation.

I claim:

1. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least percent and reduction in area at least 30 percent, essentially consisting of about 1 1 percent to about percent chromium, about 9.39 percent to about 12 percent cobalt, about 4 percent to about 8 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.10 percent, nitrogen not exceeding 0.10 percent, columbium up to about 0.50 percent, and remainder substantially all iron.

2. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 13 percent to about 15 percent chromium, about 9.39 percent to about 12 percent cobalt with the sum of the chromium and cobalt contents not exceeding about 25 percent, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.05 percent, nitrogen not exceeding 0.10 percent, columbium up to about 0.50 percent, and remainder substantially all iron.

3. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 1 1 percent to about 13 percent chromium, about 9.39 percent to about 1 1 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, about 0.01 percent to about 0.06 percent carbon, nitrogen not exceeding 0.06 percent, columbium up to about 0.50 percent, and remainder substantially all iron.

4. Martensitic stainless steel, precipitation-hardenable tostrengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 1 1 percent to about 15 percent chromium, about 9.39 percent to about 12 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.10 percent, nitrogen not exceeding 0.10 percent, nickel below 1 percent, columbium about 0.01 percent to about 0.5.percent, and remainder substantially all iron.

5. Martensitic stainless steel, precipitatiomhardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 1 1 percent to about 13 percent chromium, about 9.39 percent to about 1 1 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, about 0.01 percent-to about 0.06 percent carbon, nitrogen not exceeding 0.10 percent, nickel below 0.5 percent, silicon not over 0.4 percent, about 0.10 percent to about 0.50 percent columbium, and remainder substantially all iron. 

1. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 11 percent to about 15 percent chromium, about 9.39 percent to about 12 percent cobalt, about 4 percent to about 8 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.10 percent, nitrogen not exceeding 0.10 percent, columbium up to about 0.50 percent, and remainder substantially all iron.
 2. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 13 percent to about 15 percent chromium, about 9.39 percent to about 12 percent cobalt with the sum of the chromium and cobalt contents not exceeding about 25 percent, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.05 percent, nitrogen not exceeding 0.10 percent, columbium up to about 0.50 percent, and remainder substantially all iron.
 3. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 11 percent to about 13 percent chromium, about 9.39 percent to about 11 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, about 0.01 percent to about 0.06 percent carbon, nitrogen not exceeding 0.06 percent, columbium up to about 0.50 percent, and remainder substantially all iron.
 4. Martensitic stainless steel, precipitation-hardenable to strengths exceeding 220,000 psi, with elongation at least 10 percent and reduction in area at least 30 percent, essentially consisting of about 11 percent to about 15 percent chromium, about 9.39 percent to about 12 percent cobalt, about 4 percent to about 6 percent molybdenum, about 2 percent to about 4 percent copper, carbon not exceeding 0.10 percent, nitrogen not exceeding 0.10 percent, nickel below 1 percent, columbium about 0.01 percent to about 0.5 percent, and remainder substantially all iron. 